Information recording in write-once type media is achieved by laser induced thermal crystallization of the as-prepared amorphous film. Recording films of this type are known as phase-change media. The amorphous-to-crystalline transition temperature (hereafter "transition temperature"), that is, the temperature at which an amorphous film spontaneously and rapidly converts to the crystalline state, is a function of the film's chemical composition. A low transition temperature is an indication of high recording sensitivity and low thermal stability of the media. In designing an optical recording thin film, an optimization is made that maximizes the performance, that is, maximizes the stability to sensitivity ratio within an acceptable range.
The amorphous material is in a metastable state and may slowly convert to the stable crystallized state in extended aging at a temperature lower than its transition temperature. A disk may become unusable by random nucleation and subsequent growth of the nuclei in the unwritten area as well as by growth of the laser written spots (which already have nuclei for growth). It is likely that growth of the laser crystallized spots will occur before random nucleation and growth. Data retention may be of greater concern than the shelf life of the unwritten disk.
Optical recording media made of binary antimony-tin alloy have relatively low transition temperatures. This may be adequate for some applications, but improvement is desirable for applications where ambient temperatures exceed room temperature.
Indium can be used to enhance the stability of the amorphous phase of the SbSn alloy by increasing the transition temperature. Indium was first selected since its atomic number (49) is similar to that of antimony (51) and tin (50).
Subsequent to the discovery that indium could be used to improve the properties of the antimony-tin alloy, other antimony-tin based alloys were discovered by the present assignee. Thus, applications were filed on antimony-tin alloys containing aluminum (U.S. Pat. No. 4,798,785); zinc (U.S. Pat. No. 4,774,170); and germanium (U.S. Pat. No. 4,795,695).
The Kodak proprietary high-performance optical recording thin films of Sb-Sn-In alloys are believed (based on the extrapolation of high temperature data) to indefinitely remain in the as-prepared amorphous state at room temperature. In thermally incubated samples, however, the laser crystallized spots show extensive growth.
It would be desirable to preserve the advantageous features of SbSnIn and other SbSn alloys and increase their stability under conditions where ambient temperatures exceed room temperature.